Polyimide powder has been produced by (1) polymerizing a corresponding tetracarboxylic acid dianhydride and a diamine in an organic solvent under heating to prepare a solution of a polyamide acid which is a precursor of polyimide; (2) introducing this solution into a solvent such as water which does not solubilize the polymer; (3) collecting the resulting precipitate; (4) heating the precipitate for ring closure to form a polyimide; and (5) mechanically crushing the polyimide.
Polyimide powder has also been prepared by (1) polymerizing, under heating, a corresponding tetracarboxylic acid dianhydride and a diamine in a solvent such as ethylene glycol which does not solubilize the polymer to form a slurry of polyamide acid; (2) filtering out the polyamide acid from the slurry; (3) heating the filtered polyamide acid for ring closure to form a polyimide; and (4) crushing the polyimide.
In the above processes, polyamide acid, which is a precursor of polyimide, is prepared first and then the polyamide acid is turned into polyimide through ring closure with heating and the resulting polyimide is crushed into fine particles. These steps are employed because the direct polymerization of a tetracarboxylic acid dianhydride and a diamine does not proceed smoothly due to the formation of gel-like polymide which cannot be easily discharged and crushed.
The above-described processes are industrially disadvantageous because complex processing steps are required, the resulting powder tends to be coarse, and a special pulverizing technique is required for producing fine particles.
A process for producing polyimide powder which is fusible (or thermoplastic) and sometimes soluble in organic solvents has been developed. In this process, the polymerization reaction is carried out in a polar solvent for 3, 3', 4, 4'-benzophenonetetracarboxylic acid dianhydride and a diisocyanate mixture of tolylene diisocyanate and diphenylmethane-4, 4'-diisocyanate, to directly form a slurry or solution of polyimide. The slurry or solution of polyimide is then introduced into a solvent such as acetone or isopropyl alcohol for precipitation. The precipitate is then filtered out and crushed.
If the diisocyanate mixture in the above process described contains less than 70 mol % tolylene diisocyanate, the polymerization reaction forms the polyimide in the form of slurry. Further, if the diisocyanate mixture contains more than 70 mol % tolylene diisocyanate, the polymerization reaction forms the polyimide in the form of solution.
The above-described process is intended to prevent the formation of a gel and to produce polyimide directly by replacing the diamine component with the diisocyanate component. It is simpler than the process of forming polyamide acid and then turning it into a polyimide through ring closure with heating.
However, the above-described process, is essentially the same as the conventional process for preparing infusible and insoluble polyimide powder because it employs the indispensable steps of preparing a slurry or solution of polyimide, introducing the slurry or solution into a solvent which does not solubilize the polymer such as acetone or isopropyl alcohol, filtering the precipitate, and drying and crushing the precipitate. Moreover, such a process does not readily provide polyimide powder of fine particles.